In conventional numerical control apparatuses (in what follows, numerical control is referred to as NC), a main spindle holding a workpiece is given a constant rotation, the amount of rotation of the main spindle is detected by an encoder attached to the main spindle, threading-spindle movement amount proportional to a prescribed thread lead is computed, and movement control is carried out.
Further, when a thread is made in the workpiece, in general, thread-cut machining is repeated several times while varying tool cutting amount, and this controls starting of interpolation for the threading spindle, with a main-spindle single-rotation reference-position signal as reference. However, since the main-spindle single-rotation reference position signal and NC control cycle IT are not synchronous, in cases where two or more machining repetitions are carried out for the same thread, there have been occurrences of variations in the actual cutting position.
Further, since there are variations in the cut-finish path near the end of the threading, due to variations produced at start of threading, an incomplete thread portion at the thread ending becomes long, and it is necessary to take into account the incomplete thread length including the variation portion. Further, due to fluctuations in cutting load due to variations in the cut-finish path close to the end point of the threading, thread accuracy near the end point degrades, and due to the fluctuations in this cutting load, there have been cases where tool life has been shortened.
Using FIG. 6 to FIG. 8, a detailed description of these issues is as follows.
As illustrated in FIG. 8, thread-cut machining is implemented by giving the workpiece, attached to the main spindle, a constant rotation, and by moving a cutting tool from a threading starting position, set at a fixed prescribed position, in the direction of the threading spindle.
In conventional threading control, as illustrated in FIG. 6, the main-spindle position, when threading, is obtained by a main-spindle position counter in which, when threading, a counting operation is started with the main-spindle single-rotation reference position signal, which is output by the encoder installed on the main spindle every time the main spindle makes one rotation, as a trigger. Further, when threading, regarding this main-spindle position counter, output is generated by the main-spindle position counter that is used for control of each rotation, the count is cleared at a prescribed count number, and the count restarted. The threading control controls the starting of threading-spindle interpolation based on the main-spindle single-rotation reference position signal; at this time, the amount the threading spindle moves is computed in the interpolation processing of the NC apparatus, executed at a constant cyclic interval (for example, 10 msec), and the amount the threading spindle moves at the first interpolation is:FΔTo=(ΔPo÷P)×L    (ΔPo: first-time main-spindle position variation amount [number of pulses],    P: number of pulses per main-spindle single rotation,    L: thread leadHere, since the main-spindle rotation cycle and the control cycle IT are asynchronous, variations occur in the value of ΔPo and also in the threading-spindle movement amount, FΔTo, at the first interpolation. These variations become the variations at the start of threading, and finally, as illustrated in FIG. 8, become the variations at the start of cut-finishing, so that the cut-finish path is no longer constant. FIG. 7 illustrates how the threading-spindle movement amount, FΔT1, at the final interpolation varies when the threading-spindle movement amount, FΔTo, at the first interpolation varies.
Technology for solving these types of problems is disclosed in Japanese Laid-Open Patent Publication 1993-46236, consisting of altering the control cycle for a servo, so as to have synchronicity with the main-spindle single-rotation reference signal However, since the above technology requires special hardware (H/W) apparatus in order to change the control cycle just before the thread-cut machining, there are disadvantages in that the H/W configuration becomes complicated.